This invention relates generally to universal joints of the type having two yokes pivotally connected by means of pins to a center block.
Universal joints are used to transfer torque from a rotating input shaft to an output shaft when the centerlines of the two shafts are offset by some angle, called the offset angle. Universal joints of the pin and block type are often used for this purpose.
A universal joint of the pin and block type (hereinafter referred to as a universal joint) generally comprises two yokes, four pins, and a center block. Each yoke generally has a straight yoke shaft, often of circular cross section, having two ears at one end spaced 180 degrees apart. The ears of each yoke carry a bearing aligned on a common axis that is perpendicular to the centerline of the yoke shaft. The two yokes are displaced 180 degrees with their ears interleaved so that the axes through their respective bearings are oriented 90.degree. apart and intersect the center block, which is positioned between the ears of the respective yokes. A pin inserted through each bearing and securely fastened to the center block allows each yoke to pivot about the axis of the pins, limited only by yoke to yoke contact. Input and output shafts may be connected to the free ends of the yoke shafts.
The construction of universal joints often varies depending on the severity of service conditions.
When universal joints are used in applications where they are subjected to relatively low torque and low rotational velocities, the yokes may be positioned such that the axes through their respective bearings do not lie in the same plane. This simplifies manufacture since the axes of the pins do not intersect and a single pin can be used to extend through the center block and the bearings in each pair of yoke ears. Since the load the pins must bear is low, they can be made from a soft material such as a low carbon steel so that the entire joint assembly can be maintained by simply riveting the ends of each pin. When the bearing axes are off-set as described above, operation of the universal joint results in articulation of the centerblock. However, at low rotational velocities like those encountered in hand operated applications such as remote control linkages in awning devices and snow blowers, articulation of the center block is usually inconsequential.
When universal joints are used in high torque and high speed applications, such as machine tool drive trains, other aspects of construction are considered. At high rotational velocities center block articulation would produce undesirable vibration. For this reason the universal joints are constructed so the bearing axes in the ears of the yokes lie in the same plane, intersecting at a 90 degree angle. Since the bearings and pins must have superior load-bearing capacity to endure the high torque levels, journal pins and journal bearings are almost exclusively used. They are typically made of steel, and are heat-treated to a hardness exceeding 40 Rockwell C.
To reduce friction and wear, a uniformly distributed lubricant film, such as an oil, is desirably maintained between the journal bearings and journal pins. In most journal pin and journal bearing applications, such as railroad axles, the shaft or pin rotates 360 degrees with respect to the bearing. This complete revolution helps to continually distribute lubricant between the pin and bearing surfaces. Operation of a universal joint, however, produces oscillatory angular displacement of the journal pins with respect to the journal bearings. The magnitude of the displacement is equal to twice the offset angle and since the offset angle seldom exceeds 25 degrees, the total displacement is usually limited to about 50 degrees. This oscillatory angular displacement of limited magnitude hampers lubricant distribution and produces increased surface contact between the journal pins and journal bearings. This metal to metal contact generates substantial amounts of frictional heat and increases the wear rate of the bearings and pins. Running temperatures in the pin and bearing zone often climb high enough to vaporize or oxidize substantially all the remaining lubricant causing the pins and bearings to quickly fail by wearing, scoring or seizing. The rate of frictional heat generation is related to the operating parameters of torque level, rotational velocity and offset angle.
Universal joints encounter particularly severe service conditions when used for mechanical linkage in multiple-spindle drill head machines. Multiple-spindle drill head machines have a number of spindles driven off the same gear in the same head. Torque from the drive gear is transmitted to each spindle through an intermediate shaft having universal joint connections at both ends. Torque levels on the universal joints vary depending on the working stock and the operation performed. It is not uncommon however, for torque levels to intermittently surge. This happens when the tool "bites" the working stock as often occurs near the end of a drilling cycle as the bit breaks through the stock. Typical rotational velocities are about 1800 r.p.m., but may be as high as 2500 r.p.m. Under normal circumstances these universal joints must be replaced every three to four months and usually because of a failure related to lack of lubrication. When extremely high levels of torque are encountered, the universal joints may have to replaced as often as weekly.
Various attempts have been made to extend the life of universal joints employed in high speed and high torque applications and especially in multiple-spindle drill head machines. It is known that lubricant circulation is facilitated by maintaining dimensional tolerances on the order of 0.0002 inch on the outside diameter of the journal pin and the inside diameter of the journal bearing. Such precision machining is, however, very costly. To continually replenish the lubricant film in the universal joint, the entire universal joint is sometimes encased in a flexible boot that is filled with lubricant, but this is expensive and adds considerably to the diameter of the universal joint. Hence, where the working space is small as in a multiple-spindle drill head, it is not practical to use such large diameter universal joints encased within a lubricant-filled boot. Further, breakage of the boot and leakage of the lubricant is possible. Many multiple-spindle drill head machines have a forced spray of lubricant directed at the universal joint in the vicinity of the journal bearings. This has proved marginally successful, but high running temperatures still result in the loss of the lubricant film between the journal pins and journal bearings.